Method of producing a glass optical element capable of suppressing occurrence of a damage of a mold and improving a quality of the glass optical element

ABSTRACT

A mold has a moling surface of a material containing silicon carbide and/or silicon nitride as a main component and a carbon thin film formed on the molding surface to prevent fusion sticking. A glass substance which has a sag point not higher than 565° C. and a predetermined composition free from arsenic oxide is introduced into the mold. The glass substance is press-formed in a heated and softened condition into a glass optical element of high precision.

This is a divisional of Application Ser. No. 08/715,415 filed Sep. 18,1996 now U.S. Pat. No. 5,919,718, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

This invention relates to a method of producing a glass optical elementwith a high precision and to a glass substance processed by the methodinto the glass optical element.

A wide variety of development and research have been made about thetechnique of press-forming or press-molding a glass substance within amold to produce a glass optical element of high precision. For example,one attempt has been directed to a mold which has a molding surface ofsilicon carbide, silicon nitride, and the like.

Herein, silicon carbide and silicon nitride are excellent in hardnessand strength against a high temperature. Such a molding surface ofsilicon carbide and/or silicon nitride may be deposited by a chemicalvapor deposition (CVD) method. In this event, the molding surface has anexcellent compactness without occurrence of surface defects, such aspores, and can be polished into a mirror surface. However, the moldingsurface of such materials is liable to be oxidized to form a siliconoxide surface layer of several tens of angstroms thick. In this case, ithas been pointed out that fusion sticking often takes place between themolding surface and the glass substance during a press-forming step whenthe glass substance is composed of borosilicate glass or silicate glasscontaining a large amount of modification components, such as alkali oralkaline earth positive ions. Moreover, stress is concentrated in afollowing cooling step here and there on the molding surface, whichcauses cracks to occur on the molding surface. This results in aphenomenon such that the molding surface of the mold is scooped orremoved in spots. This phenomenon will hereinafter be simply called a"pullout" or a "pullout" phenomenon.

Taking the above into consideration, Japanese Patent Publication (B4)No. 61816/1992 and Japanese Unexamined Patent Publication (A2) No.199036/1990 disclose a method of forming a carbon thin film on themolding surface of silicon carbide or silicon nitride. The carbon thinfilm may be either a hard carbon film or an i-carbon film and serves toprevent the above-mentioned fusion sticking of the glass substance tothe mold. By coating the molding surface of silicon carbide or siliconnitride with the carbon thin film, the fusion sticking and the pulloutcan effectively be avoided. Thus, the carbon thin film is helpful torelease from the molding surface the glass substance pressed by the moldand may be called a releasing thin film.

However, it is impossible to form a perfect carbon thin film which isfree from defects and which completely covers an entire area of themolding surface of the mold. In other words, film defects aremicro-scopically observed in the carbon thin film. This fact isdisclosed in Japanese Unexamined Patent Publication (A2) No. 120245/1990and well known in the art.

If the carbon thin film has such film defects, silicon carbide orsilicon nitride of the molding surface is exposed through the filmdefects of the carbon thin film and locally oxidized to form the siliconoxide surface layer. Such local oxidization of the molding surfacecauses the pullout to occur due to both the fusion sticking of the glasssubstance to the mold and the stress concentration during repetition ofthe press-forming step and the cooling step.

In addition, the carbon thin film is not permanently invariable and maybe peeled off from the mold by repetition of the press-forming stepfollowed by the cooling step as a result of oxidization of siliconcarbide or silicon nitride, which weakens the adhesion strength of thecarbon thin film.

Taking the above into consideration, the carbon thin film is forciblydetached and removed from the molding surface after these steps arerepeated over a predetermined period. Thereafter, a new carbon thin filmis formed to reproduce or reuse the mold.

However, the film defects inevitably take place in the new carbon thinfilm. If the press-forming step is repeated by the use of the mold withthe film defects left in the carbon thin film, the pullout is caused tooccur as described in the foregoing. The spread of the pullout not onlyresults in degradation of a glass optical element but also in anunrecoverable damage of the mold.

As described above, it is technically impossible in an industrial scaleto form a non-defect carbon thin film and also to completely cover theentire area of the molding surface of the mold with such non-defectcarbon thin film.

Under the circumstances, consideration might be directed to the glasssubstance to be processed by press-forming. However, no disclosure hasbeen made yet about an effective glass substance.

Recently, Japanese Unexamined Patent Publication (A2) No. 345461/1994proposes a glass substance which includes neither arsenic oxide norantimony oxide and which can be readily press-formed. Specifically, thisproposal is based on the following facts. Namely, a dense crown glass isexemplified as a glass substance which contains by weight 0.2% arsenicoxide and 0.2% antimony oxide as a refining agent and a decoloringagent. The glass substance is placed in a mold which has a moldingsurface coated with an amorphous diamond-like carbon thin film and isheated to a temperature between 750° C. and 1250° C. In this event,reaction occurs between the carbon thin film and oxygen gas releasedfrom arsenic oxide and antimony oxide. As a consequence, the carbon thinfilm is oxidized, consumed, and partly peeled off. In order to avoidsuch reaction, arsenic oxide and antimony oxide are excluded from theglass substance proposed in the above-mentioned publication.

However, exclusion of both arsenic oxide and antimony oxide from theglass substance brings not only about deterioration of a seed-freecharacteristic of a glass melt but also about decoloration of theresultant glass article. It is therefore required to solve thesedisadvantages.

SUMMARY OF THE INVENTION

It is an object of this invention to provide a method of producing aglass optical element, which can preferably suppress a pulloutphenomenon of a mold.

It is another object of this invention to provide a glass substancewhich is capable of effectively reducing occurrence of the pullout.

It is still another object of this invention to provide a glass opticalelement which is produced from the glass substance.

Typically, a glass substance which has a predetermined glass compositionand a predetermined sag point is press-formed at a glass viscositybetween 10⁷ and 10⁹ poises.

The present inventors have investigated the cause of the reactionbetween a carbon thin film and a glass substance during thepress-forming step and the peel-off of the carbon thin film. This leadsto the findings which will presently be described. At first,consideration is made about consumption of the carbon thin filmresulting from the reaction between the carbon thin film and the glasssubstance being press-formed. At a high temperature not lower than 750°C. and a low glass viscosity not higher than 10⁵ poises, the carbon thinfilm is oxidized and consumed by oxygen released from arsenic oxide andantimony oxide, as described in the above-mentioned Japanese UnexaminedPatent Publication (A2) No. 345461/1994. In contrast, at a lowtemperature of 630° C. and within the above-mentioned high viscosityrange typically used in-the press-forming step, oxidization of thecarbon thin film by arsenic oxide and antimony oxide contained in theglass is very little as far as a nonoxidizing atmosphere is maintained.Thus, there is no significant difference between presence and absence ofarsenic oxide and antimony oxide. By repetition of the press-formingstep, silicon carbide is gradually oxidized through defective portionsof the carbon thin film to form a surface layer of silicon oxide. Inthis event, the adhesion strength between the carbon thin film and themolding surface is gradually weakened. This results in peel-off of thecarbon thin film after lapse of a predetermined time period during whichthe press-forming step is repeated. If oxygen is contained in theatmosphere, the carbon thin film is oxidized and consumed by oxygen.

The present inventors have also studied about a glass substance which iscapable of suppressing occurrence of a so-called pullout. The pullout isa phenomenon that the surface of the mold is scooped or removed in spotsas described in the preamble of the instant specification. As a result,it has been confirmed that, at a low temperature of 630° C. for example,the pullout is undesiredly caused to occur when the glass substancecontains arsenic oxide as a refining agent and a decoloring agent. Onthe other hand, inclusion of a predetermined content of antimony oxideis difficult to cause the pullout.

The above-mentioned studies by the present inventors have revealed thefollowing. Upon production of a glass optical element by the use of amold with a releasing carbon thin film formed on a molding surface toavoid fusion sticking, peel-off of the carbon thin film and a pulloutcan be effectively suppressed by the use of a glass substance which hasa low softening point to enable press-forming or press-molding at alow-temperature and which does not contain arsenic oxide as a glasscomponent. Throughout the instant specification, the term "arsenicoxide" includes As₂ O₃ and As₂ O₅.

This invention is based on the above-mentioned various studies andobservations. According to this invention, a method of producing a glassoptical element comprises the step of preparing a mold which has amolding surface of a material containing silicon carbide and/or siliconnitride as a main component. The molding surface is covered with areleasing carbon thin film which serves to avoid fusion sticking. Themethod further comprises the steps of introducing into the mold a glasssubstance which has a sag point not higher than 565° C. and which doesnot contain arsenic oxide as a glass component, and press-forming theglass substance in a heated and softened state. Herein, the "sag point"is determined in the following manner known in the art. Specifically, aload of 10 g is imposed on a glass sample of 5 mm in diameter and 20 mmin length and a thermal expansion is measured. The sag point is definedby a temperature at which an apparent thermal expansion is stopped andshrinkage is started in the glass sample. The sag point corresponds to aviscosity ranging from about 10¹⁰ to 10¹¹ poises.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the relationship between the sag points and pullout numbersof samples according to this invention and comparative examples;

FIG. 2 shows the relationship between amounts of As₂ O₃ and Sb₂ O₃ andthe pullout numbers of samples according to this invention andcomparative examples; and

FIG. 3 is a graph showing a lifetime of a mold according to thisinvention in comparison with one comparative example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In a method of producing a glass optical element according to thisinvention, use is made of a mold having a molding surface of a materialcontaining silicon carbide and/or silicon nitride as a main component.The molding surface is covered with a releasing carbon thin film whichserves to prevent fusion sticking.

The above-mentioned mold may be made of silicon carbide and/or siliconnitride as a base material. In this event, the molding surface is formedas an integral part of the mold because its material is same as the basematerial of the mold. Alternatively, when a different base material suchas cemented carbide is used, the molding surface is formed by a thinlayer of silicon carbide and/or silicon nitride deposited on the basematerial, directly or indirectly through an intermediate layer. The thinlayer of silicon carbide and/or silicon nitride is deposited by the useof a CVD method, a sputtering method, a plasma CVD method, or the like.Among those, deposition of silicon carbide and/or silicon nitride by theCVD method is preferred because of excellent compactness. Deposition ofβ-SiC by the CVD method is most preferred. It is noted here that themolding surface essentially contains silicon carbide and/or siliconnitride as a main component. Accordingly, the molding surface may bemade of a material exclusively containing silicon carbide and/or siliconnitride. Use may also be made of a ceramics material, such as siliconnitride ceramics (SiAlON), containing at least 90 wt % of siliconcarbide and/or silicon nitride.

In case where the molding surface is formed by the thin layer containingsilicon carbide and/or silicon nitride as a main component and formed onthe base material such as cemented carbide, the thickness of the thinlayer is preferably between 0.02 and 2 μm.

More preferably, the molding surface is formed by a thick layerdeposited on a sintered body of silicon carbide and/or silicon nitrideby the CVD method. Preferably, the whole of the base material isproduced by the CVD method.

On the other hand, the releasing carbon thin film formed on the moldingsurface of the mold preferably comprises a single component layer or amixture layer having an amorphous and/or diamond-like structure becauseof excellent releasability. The releasing carbon thin film may have ormay not have a C--H bond. The releasing carbon thin film is formed byany appropriate technique such as the sputtering method, the ion platingmethod, and the plasma CVD method depending on each material usedtherefor.

The releasing carbon thin film preferably has a thickness between 0.02and 1 μm. When a thickness is smaller than 0.02 μm, the releasabilitybecomes insufficient in the carbon thin film. On the other hand, thethickness over 1 μm deteriorates the surface roughness and makes itdifficult to keep an internal configuration of the mold in apredetermined shape, namely, to maintain surface accuracy.

Prior to the formation of the releasing carbon thin film, anintermediate layer may be formed on the molding surface. Alternatively,the releasing carbon thin film may be-formed by a multilayer film.

In the method of producing a glass optical element according to thisinvention, a glass substance (i) has a sag point not higher than 565° C.and (ii) contains no As₂ O₃ as a glass component. These requirements (i)and (ii) for the glass substance provides features of this invention.

The reason why the sag point is restricted in the above-mentionedrequirement (i) is as follows. As will be demonstrated in specificembodiments which will later be given, the sag point exceeding 565° C.becomes a factor of generating the pullout on the surface of the mold.On the other hand, the sag point not higher than 565° C. effectivelysuppresses the occurrence of the pullout on the molding surface toachieve an extended lifetime of the mold. When the sag point exceeds565°, the pullout frequently occurs as compared with this inventionseven if neither of As₂ O₃ and Sb₂ O₃ is contained.

The reason why no As₂ O₃ is contained as a glass component in theabove-mentioned requirement (ii) is as follows. As will be demonstratedin the specific embodiments which will later be given, inclusion of As₂O₃ is liable to frequently cause the pullout to occur when the glasssubstance is press-formed and cooled. On the other hand, exclusion ofAs₂ O₃ serves to suppress the occurrence of the pullout.

For achieving refining and decoloring effects, addition of As₂ O₃ or Sb₂O₃ is essential. In this respect, the present inventors have found outthat, in a glass substance having a sag point not higher than 565° C.and a press-forming temperature not higher than 630°, Sb₂ O₃ does notact as a pullout generation factor although As₂ O₃ does. In thisconnection, the glass substance is desired to contain Sb₂ O₃ in view ofthe refining and the decoloring effects. The content of Sb₂ O₃ ispreferably between 0.01 and 0.5 wt %, more preferably, between 0.1 and0.3 wt %.

The glass having a sag point not higher than 565° C. and containing noAs₂ O₃ preferably has a composition essentially consisting of, byweight, 28-55% SiO₂, 5-30% B₂ O₃ (SiO₂ +B₂ O₃ being 46-70% and SiO₂ /B₂O₃ being 1.3-12.0 (weight ratio)), 5-12% Li₂ O, 0-5% Na₂ O, 0-5% K₂ O(Li₂ O+Na₂ O+K₂ O being 5-12%), 0-40% BaO, 0-10% MgO, 0-23% CaO, 0-20%Sro, 0-20% Zno (BaO+MgO+CaO+Sro+ZnO being 10-44% and SiO₂ +B₂ O₃ +Li₂O+BaO+CaO being not smaller than 72%), 1-7.5% Al₂ O₃, 0-3% P₂ O₅, 0-15%La₂ O₃, 0-5% Y₂ O₃, 0-5% Gd₂ O₃, 0-3% TiO₂, 0-3% Nb₂ O₅, 0-5% ZrO₂ (La₂O₃ +Y₂ O₃ +Gd₂ O₃ being 1-15%), and 0-0.5% Sb₂ O₃.

Preferably, the above-mentioned composition essentially consists byweight of 30-55% SiO₂, 5-30% B₂ O₃ (SiO₂ +B₂ O₃ being 56-70%), 7-12% Li₂O, 0-3% Na₂ O, 0-3% K₂ O (Li₂ O+Na₂ O+K₂ O being 7-12%), 0-30% BaO, 0-5%MgO, 0-23% CaO, 0-20% SrO, 0-10% ZnO (SiO₂ +B₂ O₃ +Li₂ O+BaO+CaO beingnot smaller than 72%), 1-7.5% Al₂ O₃, 0-2% P₂ O₅, 0-10% La₂ O₃, 0-3% Y₂O₃, 0-3% Gd₂ O₃, 0-23% TiO₂, 0-2% Nb₂ O₅, 0-3% ZrO₂ (La₂ O₃ +Y₂ O₃ +Gd₂O₃ being 1-10%), and 0-0.5% Sb₂ O₃.

Generally, lowering of the sag point deteriorates the chemicaldurability of the glass substance. According to this invention, theglass substance contains 5-12 wt % Li₂ O so that the sag point islowered without deterioration of the chemical durability.

In this invention, the glass substance is preferably a dehydrated glass.obtained by removing water contained in a glass material. The use of thedehydrated glass further suppresses the occurrence of the pullout. Thedehydrated glass is produced by bubbling a glass melt with a bubblinggas to volatilize and remove the water in the glass melt. The bubblinggas preferably comprises a mixture of nitrogen and oxygen. For example,dry air is suitable because handling is easy.

The use of SOCl₂ gas increases the dehydration degree but is difficultto handle. In addition, residual sulphur and residual chlorine result inthe occurrence of the pullout.

The total content of hydroxyl groups and water molecules contained inthe dehydrated glass is preferably 50 ppm or less, more preferably 25ppm or less, in order to prevent oxidization of the mold.

The glass substance is obtained by melting a glass material such asoxide, carbonate, and nitrate. At least within a scope of thisinvention, no tendency is observed to indicate that nitrate is inferior.

The glass substance is preferably formed as a bulk preform having apredetermined weight obtained by falling the glass melt from an outletpipe of a glass melt furnace. The bulk preform may have a sphericalshape or a marble shape. Preferably, the glass melt falling down fromthe outlet pipe is received at a floating position by the use of areception plate which injects a gas. In this event, the preform has anentire surface comprising a free surface with substantially no defectsuch as wrinkling, projections, depressions, contamination, and stickingmatters. Such preform is advantageous in several respects. Specifically,the preform does not have a surface hydrate layer (acting as a factor ofoxidization of the mold) inherent to cold grinding. Secondly, thepreform is free from an adverse influence from the sticking matters orthe like which would otherwise be present. In addition, the glasssubstance can be produced from the preform at a low cost.

In the method of producing a glass optical element according to thisinvention, the above-mentioned glass substance is introduced into themold and subjected to press-forming in a heated and softened state toobtain the glass optical element.

The press-forming is preferably carried out in a nonoxidizing atmosphere(for example, N₂ or 2% H₂ +98% N₂) containing at most 15 ppm oxygen andat most 50 ppm water. The nonoxidizing atmosphere and the oxygen contentnot greater than 15 ppm are preferred because the oxygen contentexceeding 15 ppm results in oxidization and consumption of the carbonthin film, followed by oxidization of silicon carbide or silicon nitrideof the molding surface, thereby causing the pullout resulting fromfusion sticking of the glass substance. The oxygen content not greaterthan 10 ppm is particularly preferred.

The water content in the atmosphere not greater than 50 ppm is preferredbecause the water content exceeding 50 ppm promotes oxidization ofsilicon carbide or silicon nitride of the molding surface to result ineasy occurrence of the pullout while the water content not greater than50 ppm suppresses the oxidization and the resultant pullout. The watercontent in the atmosphere equal to 25 ppm or less is particularlypreferred.

In the above-mentioned method according to this invention, use is madeof the glass having a sag point not higher than 565° C. and containingno As₂ O₃. It is therefore possible to prevent surface oxidization ofsilicon carbide or silicon nitride which provides the molding surface ofthe mold. As a consequence, it is possible to remarkably reduce theprobability of peel-off of the carbon thin film and the occurrence ofthe pullout of the molding surface. Thus, the lifetime of the mold canbe considerably extended. As compared with the conventional glass havinga sag point exceeding the above-mentioned level (565° C.) and containingAs₂ O₃, the probability of occurrence of the pullout can be reduced to1/5 or less and the lifetime of the mold can be extended to 5 times ormore according to this invention.

1st Embodiment

(1) Consideration about the Sag Point

As described above, the pullout is caused to occur even if the mold hasthe releasing carbon thin film formed on the molding surface made ofsilicon carbide or silicon nitride, because silicon carbide or siliconnitride is exposed through film defects inevitably formed in thereleasing carbon thin film and reacts with the glass substance to bescooped or removed together with the glass substance pressed by themold.

In order to clearly demonstrate the effect of this invention, a mold(flat plate processed into a mirror surface) having a silicon carbidemolding surface without a releasing carbon thin film was prepared.Various glass substances were prepared and press-formed by the use ofthe mold. The relationship between the sag point of each glass and theoccurrence of the pullout was observed.

The glass substances used here had different sag points ranging between520° C. and 590° C. and contained at least 0.01% As₂ O₃. The refractiveindexes n_(d) were between 1.55 and 1.63. The Abbe numbers ν_(d) were 55or more. Particularly, those glass substances having n_(d) of 1.59 andν_(d) of 61 were mainly used.

The press-forming condition was as follows:

Glass to be Molded: preform of a marble shape with a volume of 250 mm³obtained by hot forming

Glass Mold (Flat Plate): In order to find the glass unsusceptible to thepullout even if the the molding surface is slightly oxidized, themolding surface of the silicon carbide was slightly oxidized by the useof oxygen plasma to form an oxide layer having a thickness between 30and 40 angstroms. For each glass, eight molds were used.

Atmosphere: 2% H₂ +98% N₂

Press-Forming Temperature: the temperature corresponding to a glassviscosity of about 10⁶.9 poises which is slightly low as compared withthe normal press-forming condition

Press-Forming Pressure: 120 kg/cm²

Press-Forming Period: 60 seconds

Cooling Rate: 110° C. /min

Number of Times of Press-Forming: five times for each of eight molds

The composition, the refractive index n_(d), the Abbe number ν_(d), dothe transformation point Tg, the sag point Ts, the waterproofness Dw,the acidproofness Da, the press-forming temperature (corresponding to aglass viscosity of about 10⁶.9 poises), and the average pullout numberare shown in Tables 1 and 2 for each glass used here. The averagepullout number was calculated over the eight molds after five times ofpress-forming. The relationship between the sag point and the pulloutnumber is shown in FIG. 1. From Tables 1 and 2 and FIG. 1, strongcorrelation is clearly observed between the sag point and the pulloutnumber. It is shown that the use of the glass having a low sag point,namely, the glass allowing press-forming at a low temperatureeffectively suppresses the pullout and that the glass having a sag pointnot higher than 565° C. is preferred.

As described above, it has been confirmed by the use of the mold withoutthe releasing thin film that the low sag point not higher than 565° C.effectively suppresses the occurrence of the pullout. In thepress-forming according to this invention on the other hand, use isactually made of the mold having the releasing thin film formed on themolding surface. Obviously, by the use of the glass having a sag pointnot higher than 565° C., the probability of occurrence of the pullout isremarkably reduced as compared with the above-mentioned case without thereleasing thin film. This will be described later in detail inconjunction with the third embodiment.

As seen from Table 2, a comparative glass No. 21 contains 0.1 wt % As₂O₃ and 0.2 wt % Sb2O₃. Another comparative glass No. 22 contains 0.01 wt% As₂ O₃ and 0.01 wt % Sb₂ O₃. The former contains a greater amount ofAs₂ O₃ and Sb₂ O₃ than that in the latter. The glass No. 21 uses anitrate material while the glass No. 22 does not. Nevertheless, only aslight difference in pullout number is observed from Table 2 and FIG. 1.

Glasses Nos. 9 and 10 of this invention exhibit the small pullout numberalthough the nitrate material is used and the contents of As₂ O₃ and Sb₂O₃ are relatively large.

Obviously, the use of the nitrate material and the presence of As₂ O₃and Sb₂ O₃ in the glass substance have a very little influence upon thepullout as compared with the sag point of the glass substance. It isnoted here that, since these glass samples contain at least 0.01 wt %As₂ O₃, suppression of the pullout is realized by the low sag point of565° C. or below but is insufficient.

In Tables 1 and 2, the glasses Nos. 1 to 8, 22 and 23 were prepared bythe use of a glass material of BaCO₃ for BaO. The glass Nos. 9 to 11 and21 were prepared by the use of a glass material containing 50% BaCO₃ and50% Ba(NO₃)₂ for BaO. The glass No. 23 was prepared by the use of aglass material of Sr(N₃)₂ for SrO. For other components, H₃ BO₃, SiO₂,Al(OH)₃, K₂ CO₃, Li₂ CO₃, CaCO₃, ZnO, La₂ O₃, As₂ CO₃, Sb₂ CO₃ wereused. The waterproofness and the acidproofness are represented in termsof the weight loss ratio.

                                      TABLE 1                                     __________________________________________________________________________                   Glass No.                                                      Composition    This Invention                                                 (wt %)         1   2   3   4   5   6   7                                      __________________________________________________________________________    B.sub.2 O.sub.3                                                                              22.7                                                                              22.6                                                                              22.6                                                                              21.0                                                                              22.6                                                                              21.5                                                                              23.5                                   SiO.sub.2      37.8                                                                              36.9                                                                              39.7                                                                              36.9                                                                              37.4                                                                              37.4                                                                              33.7                                   SiO.sub.2 + B.sub.2 O.sub.3                                                                  60.5                                                                              59.5                                                                              62.3                                                                              67.9                                                                              60.0                                                                              58.9                                                                              57.2                                   SiO.sub.2 /B.sub.2 O.sub.3                                                                   1.67                                                                              1.63                                                                              1.76                                                                              1.76                                                                              1.65                                                                              1.74                                                                              1.43                                   Al.sub.2 0.sub.3                                                                             5.0 5.6 5.6 4.4 5.6 6.1 7.3                                    K.sub.2 O      --  --  --  --  --  --  --                                     Li.sub.2 O     8.0 9.3 8.3 9.0 9.3 8.8 8.8                                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                          8.0 9.3 8.3 9.0 9.3 8.8 8.8                                    BaO            21.5                                                                              22.1                                                                              23.1                                                                              21.5                                                                              --  21.7                                                                              24.5                                   SrO            --  --  --  --  --  --  --                                     CaO            --  --  --  --  22.1                                                                              --  --                                     ZnO            --  --  --  --  --  --  --                                     MgO + CaO + SrO + BaO + ZnO                                                                  21.5                                                                              22.2                                                                              23.1                                                                              21.5                                                                              22.1                                                                              21.7                                                                              24.5                                   La.sub.2 O.sub.3                                                                             5.0 3.7 3.7 4.5 3.7 4.5 2.2                                    La.sub.2 O + Y.sub.2 O.sub.3 + Gd.sub.2 O.sub.3                                              5.0 3.7 3.7 4.5 3.7 4.5 2.2                                    As.sub.2 O.sub.3                                                                             0.01                                                                              0.01                                                                              0.01                                                                              0.01                                                                              0.01                                                                              0.01                                                                              0.01                                   Sb.sub.2 O.sub.3                                                                             --  --  --  --  --  --  --                                     Refractive Index n.sub.d                                                                     1.5883                                                                            1.5892                                                                            1.5889                                                                            1.5891                                                                            1.6030                                                                            1.5887                                                                            1.5891                                 Abbe Number ν.sub.d                                                                       61.4                                                                              61.0                                                                              61.3                                                                              61.0                                                                              59.6                                                                              60.9                                                                              61.1                                   Transformation Point (°C.)                                                            512 500 507 504 498 500 495                                    Sag Point (°C.)                                                                       553 537 546 543 538 537 529                                    Waterproofness Dw (wt %)                                                                     0.21                                                                              0.30                                                                              0.26                                                                              0.29                                                                              0.10                                                                              0.17                                                                              0.17                                   Acidproofness Da (wt %)                                                                      0.80                                                                              0.80                                                                              0.83                                                                              0.83                                                                              0.67                                                                              0.57                                                                              0.77                                   Press-forming Temperature                                                                    612 597 607 606 595 600 600                                    (°C.)                                                                  Average Pullout Number                                                                       10.8                                                                              7.8 7.8 11.1                                                                              4.8 7.0 4.3                                    __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________                   Glass No.                                                      Composition    This Invention  Comparative Sample                             (wt %)         8   9   10  11  21  22  23                                     __________________________________________________________________________    B.sub.2 O.sub.3                                                                              27.4                                                                              24.0                                                                              11.5                                                                              24.0                                                                              19.5                                                                              19.5                                                                              20.0                                   SiO.sub.2      39.4                                                                              37.8                                                                              47.2                                                                              38.2                                                                              39.4                                                                              39.4                                                                              39.2                                   SiO.sub.2 + B.sub.2 O.sub.3                                                                  66.8                                                                              61.8                                                                              58.7                                                                              62.2                                                                              58.9                                                                              58.9                                                                              59.2                                   SiO.sub.2 /B.sub.2 O.sub.3                                                                   1.44                                                                              1.58                                                                              4.10                                                                              1.59                                                                              2.02                                                                              2.02                                                                              1.96                                   Al.sub.2 O.sub.3                                                                             5.0 5.0 3.2 5.5 5.0 5.0 6.0                                    K.sub.2 O      --  --  1.5 --  --  --  --                                     Li.sub.2 O     7.0 8.5 7.3 8.0 5.0 5.0 5.3                                    Li.sub.2 O + Na.sub.2 O + K.sub.2 O                                                          7.0 8.5 8.8 8.0 5.0 5.0 5.3                                    BaO            --  16.1                                                                              21.8                                                                              12.6                                                                              25.4                                                                              24.5                                                                              8.5                                    SrO            --  --  --  --  --  --  13.7                                   CaO            17.5                                                                              5.0 --  7.3 2.0 2.0 3.6                                    ZnO            --  --  5.0 --  --  --  --                                     MgO + CaO + SrO + BaO + ZnO                                                                  17.5                                                                              21.1                                                                              26.8                                                                              19.9                                                                              27.4                                                                              26.5                                                                              25.8                                   La.sub.2 O.sub.3                                                                             3.0 3.0 2.0 4.1 3.0 3.0 3.0                                    La.sub.2 + Y.sub.2 O.sub.3 + Gd.sub.2 O.sub.3                                                3.0 3.0 2.0 4.1 3.0 3.0 3.0                                    As.sub.2 O.sub.3                                                                             0.01                                                                              0.10                                                                              0.50                                                                              0.01                                                                              0.10                                                                              0.01                                                                              0.01                                   Sb.sub.2 O.sub.3                                                                             0.01                                                                              0.20                                                                              --  0.01                                                                              0.20                                                                              0.01                                                                              0.01                                   Refractive Index n.sub.d                                                                     1.5907                                                                            1.5891                                                                            1.5831                                                                            1.5891                                                                            1.5891                                                                            --  1.5891                                 Abbe Number νd                                                                            61.4                                                                              61.3                                                                              59.5                                                                              61.3                                                                              61.3                                                                              --  61.3                                   Transformation Point (°C.)                                                            530 502 498 514 545 542 536                                    Sag Point (°C.)                                                                       565 539 538 545 587 584 573                                    Waterproofness Dw (wt %)                                                                     0.11                                                                              0.11                                                                              0.04                                                                              0.08                                                                              0.08                                                                              0.09                                                                              0.08                                   Acidproofness Da (wt %)                                                                      0.81                                                                              0.83                                                                              0.36                                                                              0.70                                                                              0.82                                                                              0.83                                                                              0.75                                   Press-forming Temperature                                                                    630 605 615 615 670 670 650                                    (°C.)                                                                  Average Pullout Number                                                                       14.8                                                                              4.5 8.7 10.0                                                                              28.7                                                                              26.8                                                                              23.3                                   __________________________________________________________________________

(2) Consideration about the Content of As₂ O₃ and Sb₂ O₃ As₂ O₃ and Sb₂O₃ are contained in the glass composition as a refining agent. Themechanism of the refining agent will presently be described inconjunction with As₂ O₃ by way of example. In a glass melting step, As₂O₃ takes O₂ from nitrate to form As₂ O₅. In a subsequent clarifyingstep, As₂ O₅ releases O₂ to thereby act as the refining agent.Advantageously, As₂ O₃ can take excessive O₂ again in a later coolingstep. This also applies to Sb₂ O₃.

Consideration will herein be made of the influence of the contents ofAs₂ O₃ and Sb₂ O₃ upon the occurrence of the pullout.

In order to examine whether or not silicon carbide is oxidized byinclusion of As₂ O₃ and Sb₂ O₃ to cause the pullout to occur, use wasmade of the mold (silicon carbide) from which the oxidized surface layerwas removed by the use of hydrogen fluoride aqueous solution.

At first, eleven samples (a) to (k) of hot-formed preforms of a marbleshape were prepared with a basic composition similar to the glass No. 11(having a sag point of 545° C.) in Table 2 except that the contents ofAs₂ O₃ and Sb₂ O₃ are different. For the eleven samples (a) to (k), thecontents of As₂ O₃ and Sb₂ O₃ and the coloring degrees are shown inTable 3 and the press-forming conditions are shown in Table 4.

                  TABLE 3                                                         ______________________________________                                                                         Coloring Degree                                       As.sub.2 O.sub.3                                                                      Sb.sub.2 O.sub.3                                                                      Material                                                                              (.sub.T  = 80%/.sub.T  = 5%)                 Sample No.                                                                             (wt %)  (wt %)  for BaO (nm)                                         ______________________________________                                        (a)      0       0       BaCO.sub.3                                                                            384/289                                      (b)      0.01    0.01            348/284                                      (c)      0.06    0.06            347/284                                      (d)      0.15    0.15            350/286                                      (e)      0.20    0.20            351/286                                      (f)      0       0.12            346/286                                      (g)      0       0.30            346/291                                      (h)      0       0.40            346/291                                      (i)      0       0       Ba(NO.sub.3).sub.2                                                                    380/287                                      (j)      0.15    0.15            353/287                                      (k)      0       0.30            343/286                                      ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Press-Forming Temperature                                                                        615° C.                                             Press-Forming Load 110 kg/cm.sup.2                                            Press-Forming Period                                                                             600 sec                                                    Cooling Rate       -110° C./min                                        Atmosphere         N.sub.2  + H.sub.2  (dry column used)                      Number of Times of five                                                       Press-Forming                                                                 Preform            hot-formed article                                                            of a marble shape                                          Preprocessing of Mold                                                                            acid treatment                                             Number of Molds    eight flat-plate mold per                                                     each condition                                             ______________________________________                                    

The relationship between the content (wt %) of As₂ O₃ and/or Sb₂ O₃ andthe pullout number after five times of press-forming is illustrated inFIG. 2. As seen from FIG. 2, the samples (f), (g), and (h) containingSb₂ O₃ without As₂ O₃ exhibit the pullout number as small as one, two,and two, respectively. In contrast, the samples (b), (c), (d), and (e)containing both As₂ O₃ and Sb₂ O₃ at a ratio of 1/1 exhibit the pulloutnumber as large as five or six. In other words, by the use of the glasssubstance containing no As₂ O₃, it is possible to remarkably reduce thepullout number.

The samples (i), (i), and (k) using Ba(NO₃)₂ as the material for BaOshow the result substantially similar to that of the samples (a) to (h)using BaCO₃ as the material for BaO.

On the other hand, consideration will be made about the case where theglass No.. 22 (having a sag point of 587° C. beyond the range specifiedin this invention and a press-forming temperature of 670° C.) is adoptedas a basic glass composition. As obvious from FIG. 2, the sample (1)containing neither As₂ O₃ nor Sb₂ O₃ and the samples (m) and (n)containing Sb₂ O₃ without As₂ O₃ exhibit the pullout number as many as5.5 and 10, respectively. Thus, the occurrence of the pullout isdistinct. The samples (o), (p), and (q) containing As₂ O₃ and Sb₂ O₃ ata ratio of 1/1 exhibit further distinct occurrence of the pullout.

From the above-mentioned results, it has been found out that even alittle amount of As₂ O₃ seriously adversely affect the occurrence of thepullout while a little amount of Sb₂ O₃ brings about no problem as faras the sag point falls within the range specified in this invention.

Second Embodiment

In order to examine the influence of the water contained in the glasssubstance, the dehydrated glass was prepared as follows. Preparation wasmade of a glass material having a basic composition similar to the glassNo. 1 in Table 1 except that no As₂ O₃ was contained but 0.12% Sb₂ O₃was contained. During the melting step, a mixture gas of 80 vol % N₂ and20 vol % O₂ was made to pass through the glass melt for bubbling toremove the water contained therein. The dehydrated glass thus obtainedhad a water content of 14 ppm. For comparison, the undehydrated glasssubjected to no dehydration process was prepared and had a water contentof 95 ppm. The mold was subjected to acid treatment in the mannersimilar to that mentioned above to leave substantially no oxide layer onthe molding surface. The press-forming atmosphere having the watercontent not greater than 50 ppm was produced by removing the excessivewater. Under the conditions defined in Table 3, the press-forming wascarried out five times for each of the dehydrated glass and theundehydrated glass. As a result, the pullout number was 1.2 in theundehydrated glass having the water content of 95 ppm while the pulloutnumber was reduced down to 0.6 in the dehydrated glass having the watercontent of 14 ppm. Further studies have revealed that the pullout isvery effectively reduced when the total content of hydroxyl groups andwater molecules contained in the glass substance is 50 ppm or less interms of water molecules.

Third Embodiment

Description will be made as regards an embodiment of practically forminga double-convex lens having an outer diameter of 12 mm.

Upper and lower molds having molding surfaces made of silicon carbidedeposited by the CVD method were prepared and finished. On each of themolding surfaces, an i-carbon film was formed by the ion plating methodto the thickness of 500 angstroms. The upper and the lower molds havingthe concave molding surfaces were combined with a columnar mold to forma complete mold. Ten sets of such molds were prepared for each of thesample of this invention and one comparative example. The glass of thesample of this invention had a basic composition similar to the glassNo. 11 (having a transformation point of 514° C. and a sag point of 545°C.) in Table 2 except that no As₂ O₃ was contained but 0.2% Sb₂ O₃ wascontained. The glass was melted into the glass melt.

The glass melt was made to fall down from the outlet pipe and receivedat a floating position by the use of a reception plate injectingnitrogen gas. Thus, the preform of a marble shape was obtained by hotforming. On the other hand, the comparative example had a glasscomposition similar to the glass No. 22 in Table 2 (having atransformation point of 514° C. and a sag point of 545° C.) with 0.1%As₂ O₃ and 0.2% Sb₂ O₃ contained. In the manner similar to thatdescribed above, the preform of a marble shape was obtained by hotforming.

A press-forming apparatus was kept at an atmosphere which contained 2%H₂ and 98% N₂ and which was dehydrated by the dry column. The preformwas placed in the mold and subjected to heating, press-forming, cooling,and removal. The series of those steps were repeated. The press-formingwas carried out at a press-forming temperature corresponding to a glassviscosity of 10⁷.8 oises. The press-forming temperature was equal to592° C. and 645° C. in the sample of this invention and in thecomparative example, respectively.

For each mold, the i-carbon film was removed at every 300 times of thepress-forming and a new i-carbon film was formed. After repeating thepress-forming for each of ten molds (twenty molding surfaces), thepullout was finally caused to occur and transferred onto the surface ofthe lens which therefore has a defective appearance. In this event, themold can not be used any longer. The number of times of thepress-forming repeated before the mold can not be used any longer isshown in FIG. 3 as the lifetime of the mold. As seen from FIG. 3, thelifetime of the mold in the comparative example ranged between 4000 and8000 times. On the other hand, in the sample of this invention, thelifetime was remarkably improved to a range between 30000 and 90000times. If the dehydrated glass is used, the lifetime of the mold isfurther improved.

In the above-mentioned method of this invention, the peel-off of thecarbon thin film formed on the molding surface and the occurrence of thepullout of silicon carbide and/or silicon nitride can be effectivelysuppressed. Therefore, the mold has an extended lifetime and a resultantglass optical element has a high quality with a reduced number ofdefects..

What is claimed is:
 1. A method of producing a glass optical elementcapable of suppressing occurrence of damage to a mold, which includessilicon in a molding surface, due to pullout caused by adherence betweenthe mold with silicon and the glass. comprising the steps of:preparing amold which includes silicon at least in a molding surface; preparing aglass substance which has a sag point not higher than 565° C. and whichis substantially free from arsenic oxide; and press molding the glasssubstance into the glass optical element with the pullout phenomenonsuppressed.
 2. A method as claimed in claim 1, wherein the moldingsurface of the mold includes, as a main component, SiC and/or Si₂ N₄. 3.A method as claimed in claim 1, wherein the molding surface is coveredwith a carbon thin film.
 4. A method as claimed in claim 2, wherein themolding surface is covered with a carbon thin film.
 5. A method asclaimed in claim 1, wherein the press molding is carried out at a presstemperature not higher than 630° C.
 6. A method as claimed in claim 1,wherein the glass substance includes SiO₂.
 7. A method as claimed inclaim 6, wherein the glass substance further includes Li₂ O.
 8. A methodas claimed in claim 6, wherein the glass substance includes, by weight,28-55% SiO₂ and 5-30% B₂ O₃,wherein SiO₂ +B₂ O₃ being 46-70%, and theweight ratio SiO₂ /B₂ O₃ being 1.3-120.
 9. A method as claimed in claim8, wherein the glass substance further includes, by weight, 5-12% Li₂ O.10. A method as claimed in claim 9, wherein the glass substance furthercomprises, by weight, 0-5% Na₂ O, 0-5% K₂ O, Li₂ O+Na₂ O+K₂ O being5-12%, 0-40% BaO, 0-10 MgO, 2-23% CaO, 0-20% SrO, 0-20% ZnO,BaO+MgO+CaO+SrO+ZnO being 10-44%, SiO₂ +B₂ O₃ +Li₂ O+BaO+CaO being notsmaller than 72%, 1-7.5% Al₂ O₃, 0-3% P₂ O₅, 0-15% La₂ O₃, 0-5% Y₂ O₃,0-5% Gd₂ O₃, 0-3% TiO₂, 0-3% Nb₂ O₅, 0-5% ZrO₂, La₂ O₃ +Y₂ O₃ +Gd₂ O₃being 1-15%.
 11. A method as claimed in claim 10, wherein the glasssubstance further comprises Sb₂ O₃ as a refining agent.
 12. A method asclaimed in claim 11, wherein the glass substance comprises, byweight,0-0.5% Sb₂ O₃.
 13. A method as claimed in claim 1, wherein theglass substance is formed by dehydrated glass.
 14. A method as claimedin claim 6, wherein the glass substance is formed by dehydrated glass.15. A method as claimed in claim 9, wherein the glass substance isformed by dehydrated glass.
 16. A method as claimed in claim 13, whereina total content of hydroxyl groups and water molecules contained in thedehydrated glass is not higher than 50 ppm.
 17. A method as claimed inclaim 14, wherein a total content of hydroxyl groups and water moleculescontained in the dehydrated glass is not higher than 50 ppm.
 18. Amethod as claimed in claim 15, wherein a total content of hydroxylgroups and water molecules contained in the dehydrated glass is nothigher than 50 ppm.
 19. A method as claimed in claim 9, wherein theglass substance has a press temperature not higher than 630° C.
 20. Amethod as claimed in claim 10, wherein the glass substance has a presstemperature not higher than 630° C.
 21. A method as claimed in claim 1,wherein the glass substance has no surface hydrate layer.
 22. A methodas claimed in claim 9, wherein the glass substance has no surfacehydrate layer.
 23. A method as claimed in claim 11, wherein the glasssubstance has no surface hydrate layer.
 24. A method as claimed in claim1, wherein the glass substance preparing step comprises the stepsof:causing glass melt of a predetermined weight to flow down through anoutlet pipe of a glass melting furnace; forming a bulk preform from saidglass melt; and rendering the bulk preform into said glass substance.25. A method as claimed in claim 24, wherein the bulk preform formingstep comprises the steps of:dripping said glass melt over a receptionplate through the outlet pipe; and floating the glass melt on thereception plate by injecting a gas through the reception plate to obtainthe bulk preform.
 26. A method as claimed in claim 25, wherein the glasssubstance is press molded within non-oxidizing atmosphere.
 27. A methodas claimed in claim 26, wherein the non-oxidizing atmosphere includes awater content not greater than 50 ppm.
 28. A method as claimed in claim27, wherein the press molding is carried out at a range between 10⁷ and10⁹ and poises of the glass substance.
 29. A method as claimed in claim28, wherein the press molding is carried out by introducing the glasssubstance into the mold with the glass substance softened.